There's a name for this phenomenon (A>B>C flights sometimes being cheaper than B>C flights using the same seats on the same plane) and there was an earlier question about it with a very detailed answer from an industry insider... unfortunately I can't remember either :-/ can anyone else remember the question I'm thinking of?
– user568458Jan 14 '16 at 11:02

1

The airline charges the maximum the market will tolerate, independent of the cost of running the service. Companies in London (on average) are willing to pay a lot more than companies in Dublin.
– CalchasJan 14 '16 at 14:13